Non-aqueous electrolyte solutions containing a lithium salt are generally used in lithium ion batteries, lithium ion capacitor and electrolytic capacitors. The non-aqueous electrolyte solution comprises an aprotic polar solvent and a lithium salt dissolved therein, as carbonate group such as ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate or methyl ethyl carbonate, butyl carbonate, or lactones such as γ-butyrolactone, or ethers such as tetrahydrofuran, and formulated two kinds of them or more. These organic solvents, however, are generally volatile and inflammable, and their use lead out safety issues whenever the battery is overly charged, discharged or short-circuited. Also liquid electrolyte solutions encounter handling difficulties when sealing the battery cells in saturating electrolyte density. The electrolyte solution may be solidified into a gel but problems of volatilization and inflammability of the organic solvent as well as leakage of liquid electrolyte made a phase separation from gel still remain unsolved.
Recently a lithium ion battery has been proposed which utilizes a non-aqueous electrolyte comprising an ambient molten quaternary ammonium salt and a lithium salt dissolved therein. See, JP-A-10/92467, JP-A-10/265674, JAP-A-11/92467, JP-A-2002/042870. The molten salt is liquid at ambient temperature but safe by properties of its non-volatility and non-inflammability. Even the molten salt electrolyte provide solidified with a matrix polymer into a gel, the resulting gel possesses poor mechanical properties and often suffers from phase separation into liquid. Thus problems remain unsolved with respect to handling of electrolyte and battery design utilizing such as molten salt electrolyte. JP-A-10/83821 and JP-A-2000/11753 propose a total solid polymer electrolyte prepared by polymerizing an ion conductive molten salt monomer having a vinyl group introduced into an imidazolium salt. This polymer electrolyte does not possess a sufficient mechanical strength and also sufficient ion transfer coefficient, that is, practically effective ionic transport number.
PCT-WO2004/88671 propose a composite polymer electrolyte composition prepared by polymerizing like grafting the electrochemically inert polymeric reinforcing material such as polyvinylidene fluoride with the molten salt polymer having a quaternary ammonium salt structure comprising quaternary ammonium cation group and anion group containing halogen atom, and also having introduced therein a plurality of crosslinkable functional groups such as carbon-to-carbon double bonds.
These grafting polymer of composite electrolyte polymer composition are slightly improved an ion conductivity and excellent non-inflammability, however, its ion transfer coefficient, that is, ionic transport number is still insufficient to meet an implementation performance level. These facts make clear in data of comparison tests described later.